A truck wheel bearing hub encloses a chamber that is partially filled with oil so that at least the bottom portion of the wheel bearing is immersed in oil when the truck is stationary. The wheel bearing is lubricated when oil is splashed due to the rotation of the wheel when the truck is moving. An access hole is provided in the center of the axial outer portion of the hub cap assembly and the oil level is usually just below the bottom of that access hole when the truck is stationary. The access hole has a plug, usually a deformable plug, usually having a vent hole therethrough, which can be readily removed. Although the plug is usually made of opaque material, the axial end wall of the hub, or at least a portion thereof, is made of a transparent material so that the truck operator, when checking the wheels at an inspection stop, can view the oil level. It is of course a sign of impending trouble if there are metallic particles in the oil of the truck hub.